In Unity 4, you could easily duplicate baked lightmapped objects and they would still share their lightmap with the original and save lightmap textures and size. In newer Unity versions however, if you try to duplicate a lightmapped object, the duplicate won’t have any lightmapping.

With this Plugin, you can conveniently duplicate lightmapped objects with CTRL + SHIFT + D and reuse them – they keep the original objects lightmap, so you can easily save on the lightmap size and don’t have to rebake all the time.

When you re-bake your original GameObject, all the duplicates will also have the new lightmap information.